High production of methylmercury in the anoxic waters of the Black Sea

As part of the GEOTRACES MedBlack cruise, the research vessel Pelagia occupied 12 full-depth stations in the Black Sea along an East-West transect between July 13th and 25th, 2013. In the permanently anoxic waters of the Black Sea, a high fraction (up to 57%) of total mercury (HgT) was found to be methylmercury (MeHg). These levels are comparable to oxic open-ocean subsurface maxima. Using a 1D numerical model, the authors demonstrated that MeHg inputs from rivers, the Mediterranean Sea and sediments are negligible and that MeHg is produced in situ in the anoxic waters. The authors also reported an increasing trend of HgT and MeHg concentrations in the anoxic waters. The numerical modeling suggests that more drastic reductions of Hg emissions are required to reach decreasing Hg and MeHg levels in the Black Sea.